1. Field of the Invention
The present invention relates generally to the use of optical fibers in data storage and retrieval systems and more particularly to the use of single-mode polarization maintaining optical fibers in magneto-optical data storage and retrieval systems.
2. Background Art
In today's technological society, as the amount of information continues to grow, storage and retrieval of the information will play an increasingly important role. In a particular information storage technology known as magneto-optical (MO) data storage and retrieval, a long term goal continues to be improved access to this information. Information access includes the use of a polarized laser light source for reading and/or writing information at a mark of interest on an MO disk. In the case of reading information, MO technology makes use of a magneto-optical effect ("Kerr" effect) to detect a polarization rotation imposed on a linearly polarized incident laser beam by a surface recording layer at the mark of interest. The polarization rotation (representing the information stored at the mark of interest) is embodied in a reflection of the linearly polarized laser beam and is converted by electronics for readout. Consequently, to accurately read stored information from an MO disk, the polarization orientation of the reflected laser beam should be faithfully conveyed from the MO disk to the readout electronics.
In one prior art proposal, S. Renard and S. Vallette (SPIE Vol. 1499, Optical Data Storage 1991, pp. 238-247) disclose an MO head design that requires three optical fibers to read and write information. Renard's MO head design is made undesirably complex, primarily, because of the large number of optical and compensating elements used in its implementation.
An alternative approach uses three polarization maintaining (PM) optical fibers. In this approach, the intrinsic properties of the fiber can be made to preserve the optical polarization as required for MO recording. Because PM optical fiber generally exhibits birefringence, (i.e., a different refractive index that different polarization orientations experience), external stresses or temperature variations may function to induce unwanted phase fluctuations between the two polarization modes of the PM optical fiber. Consequently, any information conveyed by the polarization rotation as it propagates through the PM optical fiber may also be affected. A proposal for passively eliminating phase fluctuations caused by the properties of PM optical fiber is discussed by M. N. Opsasnick in SPIE Vol. 1499, Optical Data Storage 1991, pp. 276-278. As in the design of Renard and Vallette, the Opsasnick MO head and actuator arm design is limited by its physical size, mass, and the number of optical elements required. In general, the greater the number and mass of the optical elements used to access information in an MO data storage and retrieval system, the slower the speed at which the information may be accessed, the lower the tracking bandwidth becomes, and the lower the track density that may be read or written.
A third approach to data storage based on flying head optical technology with free-space optical propagation to and from the head is proposed by N. Yamada (U.S. Pat. No. 5,255,260). In particular, Yamada discloses an optical head arrangement that requires one stationary laser/detector package per head, with the head placed on a linear actuator for movement across a disk surface. Yamada does not address the problems associated with vertical runnout of the disk or the associated degradation of the optical spot size. Although Yamada provides access to a plurality of phase change optical disks, the number of optical disks that may be operated within a given volume, as well as the performance characteristics associated with the optical disks, is inherently limited by the excessive number, size, and cost of the required optical and mechanical components.
What is needed is an optical system and method that improves upon the prior art efforts directed towards data access. Compared to the prior art, the optical system and method should preferably reduce head weight and size, improve disk access time, require fewer optical components, increase the number of storage disks that may be operated within a given volume, and be inexpensive and easy to manufacture.